The present invention relates to a Radio Frequency coil (xe2x80x9cRF chokexe2x80x9d); and more specifically, to an improved RF choke with improved RF performance and improved method for implementing the RF choke in a cable television (xe2x80x9cCATVxe2x80x9d) system carrying both an AC signal and RF frequencies.
It is within the general knowledge of those skilled in the art that RF signals and an AC power signal are commonly transmitted from the headend to the subscriber, over coaxial cable, in a typical CATV system. The AC power signal is used to power electronic devices along the network, such as amplifiers. Thus, prior to being received by the subscriber, it is customary to separate the RF signals from the AC power signal (xe2x80x9cAC signalxe2x80x9d). It is known in the art that RF chokes are used to separate (xe2x80x9cbypassxe2x80x9d) the AC signal from the RF signals. The AC power signal typically ranges in frequency from 50 to 60 Hz and typically carries from 8 to 25 Amperes (xe2x80x9cAmpsxe2x80x9d). The RF signals transmitted from the headend typically range from 50 MHz to upward of 870 MHz. It is also commonly known in the art that in systems allowing a subscriber to transmit RF signals to the headend, typically from 5 MHz to 42 Mhz and above, such RF signals may be combined with the AC signal prior to reaching the headend.
There are several problems with using RF chokes to bypass the AC signal. The first problem is the high currents being bypassed by the RF chokes creates hum distortion, which distorts the information contained within the RF signals. Thus, there is a need in the art to diminish this distortion and allow the RF signals to pass with minimum interference.
Handling the currents that rise up to 25 amps, while effectively maintaining the RF signals from 5 MHz to above 870 MHz, increases the requirements of the RF choke. For example, an RF choke which must handle currents rising up to 25 Amps, must be wrapped with a wire conductor of sufficient size (up to 14 AWG). The increased wire size increases the RF choke cost and degrades its performance. Thus, there is also a need to provide RF chokes whose increased wire size minimally effect distortion of the RF signals.
A Printed Circuit Board (xe2x80x9cPCBxe2x80x9d) is used to make the electrical connections at and around the juncture used for separating the AC current from the RF signals in most CATV systems. The CATV cable carrying both the AC signal and the RF signals is connected at a node on the PCB. After separation, the RF signals are connected to a matching network (xe2x80x9ctuning networkxe2x80x9d); whereas, the AC signal is connected to the power network through the RF choke. The connection from the CATV cable to the RF choke has typically been made by using electrical conducting traces on the PCB. Due to the high circuit current received by the RF choke, the trace leading from the CATV cable to the xe2x80x9chot sidexe2x80x9d of the RF choke had to be of sufficient size to carry up to 25 amps. This large electrical trace distorts the RF signals. Specifically, the trace interferes with precision RF adjustments of the tuning network. Thus, there is a further need in the art to avoid this high current carrying trace which interferes with RF precision tuning.
Generally RF chokes, utilize the combination of clockwise windings connected to a series of counterclockwise windings around a core, and a dampening resistor interposed between a predetermined winding amongst the counterclockwise windings and the last of the counterclockwise winding. The clockwise windings are connected to the xe2x80x9chotxe2x80x9d lead of the RF choke, which is connected to the CATV cable carrying both the RF signals and the AC signal. The general design, while suitable for its intended purpose, creates distortion with the RF signals. Thus, there is a need in the art for a revised RF choke design to help diminish the distortion and allow the RF signals to pass with minimum interference.
Prior art RF chokes, which utilized increased wire size to handle the high AC signal currents, introduced more parasitic capacitance. Thus, there is also a need in the art to provide RF chokes whose increased wire size minimally effect distortion of the RF signals.
The invention is a modified RF choke that is implemented in an innovative manner in an RF CATV circuit. In the RF CATV circuit, the RF choke is used to separate the AC signal from the RF signal in an RF circuit. The RF choke has a core that is normally made of a ferrite material. The core is usually cylindrical in shape, and has two ends. On the first end, a conductor is wrapped in a clockwise direction around the core and is considered the first winding. The first lead of the RF choke is electrically connected at the first end of the first winding. The number of times the first winding is wrapped around the core is the number of first winding turns. Hereinafter, the number of first winding turns is referred to as xe2x80x9cN1xe2x80x9d. In the newly designed RF choke, the first winding implements a spacing (xe2x80x9coffsetxe2x80x9d) between a predetermined number of first winding turns. The predetermined number of first winding turns is hereinafter referred to as xe2x80x9cXxe2x80x9d.
The second end of the first winding is connected to the second winding, which circumvents the core in the counter clockwise direction. The number of times the second winding wraps around the core is the number of second winding turns . Hereinafter the number of second winding turns is referred to as xe2x80x9cN2xe2x80x9d. The second lead is electrically connected to the second end of the second winding. In addition, there is a dampening resistor that is electrically connected between the second lead and a predetermined place amongst the second winding.
The present invention, the improved RF choke, may be implemented in an CATV system which carries the RF signal and the AC signal. The RF choke will be used to isolate the AC signal from the RF signal. The electrical connections are implemented on a printed circuit board (xe2x80x9cPCBxe2x80x9d). A CATV cable is connected to a node on this PCB. The first lead of the RF choke and a RF jumper conductor that carries the isolated RF signal to a tuning network, is also connected at this node. The AC signal goes through the RF choke to a power network. The RF signal goes through the RF jumper conductor to a connected RF tuning network. The RF jumper conductor may be used to improve RF tuning and impedance matching in the RF tuning network.